Petrology of the Proterozoic mafic dyke swarms of Uruguay and constraints on their mantle source composition

Abstract

Three dyke swarms in Uruguay range in age from Palaeoproterozoic (1.86 Ga, Florida region) to Neoproterozoic (∼0.7 Ga, Nico Perez and Treinta y Tres regions). The Florida and Nico Perez swarms are basalts, basaltic andesites and andesites with tholeiitic affinity, characterized by LILE and LREE enrichment with respect to HFSE, Nb depletion with respect to K and La, K Rb < 260 , Ba Rb < 19 , Zr Nb > 13 , (La Yb) n > 4.5 . Both swarms have positive ϵ Sr and negative ϵ Nd, but the Florida isotopic array is dominated by ϵ Nd variations (EM1 type) and that of Nico Perez by ϵ Sr variation (EM2 type). The Treinta y Tres swarm consists of slightly ne-normative transitional or alkaline basalts with geochemical characteristics resembling those of OIB (e.g., Gough Island), but suggestive of a less enriched OIB source, and have slightly positive ϵ Nd and ϵ Sr. Crustal contamination does not appear to be important in the petrogenesis of the swarms. It is proposed that the geochemical characteristics of the Florida swarm derive from melting of lithospheric mantle infiltrated by hydrous fluids released from the thermal breakdown of hydrous phases, either contained in underplated oceanic crust or formed by interaction of asthenospheric fluids with the lithospheric mantle. The EM1-type isotopic features are considered as unrelated with possible fluid addition, but as a pre-existing feature. The geochemical and isotope characteristics of the Nico Perez swarm are attributed to time-integrated enrichment processes, which affected the Nico Perez lithospheric mantle during the Palaeoproterozoic Florida episode. The geochemistry of the Treinta y Tres swarm may be explained by partial melting of a lithospheric mantle which did not suffer Palaeoproterozoic LILE and LREE enrichment but which was isotopically reset in Palaeoproterozoic times. Alternatively, a residual mantle related to the Florida event is required. Mantle melting and dyke intrusion probably occurred in an ensialic environment. The geochemical and isotopic characteristics of the Uruguay dyke swarms are similar to those of the Mesozoic basalts related to the Gondwana break-up, supporting the possibility that the latter derived from a heterogeneous lithospheric mantle source which recorded Proterozoic enrichment processes.